Sol-gel processing is a widely known method for the chemical synthesis of inorganic oxide compounds, including metal oxides and complex metal oxides. The sol-gel process is based on a theory that components in a solution are uniformly distributed. Generally, in the process, a metal alkoxide compound as a reactant is dissolved in an alcoholic solvent. The solution is then hydrolyzed by the addition of water and condensed to thereby obtain a sol-phase where particles are uniformly dispersed. Thereafter, the sol-phase is allowed to stand to obtain a gel-phase. When the reaction conditions are appropriately controlled, the resulting product may be endowed with a particle size ranging from several tens to several hundreds of nanometers. The gel-phase product has a three-dimensional metal oxide lattice structure where the particles of the inorganic components are chemically crosslinked with each other.
The sol-gel process has a number of advantages in that various types of products such as lenses, thin films, bulk particles and fibers can be prepared by the inorganic polymerization. However, the sol-gel process has serious limitations that do not easily allow its application to a practical use. For instance, it is difficult to obtain a desired shape from the sol-gel process because shrinkage, fracture or some form of distortion may easily occur during the process. Moreover, the final product has poor mechanical strength, causing mechanical failure of the shape (i.e. breakage) and poor solvent resistance, further resulting in denaturation of the shape. In particular, mechanical processing steps such as grinding, cutting and casting cannot be performed on the product due to its weak structure. Therefore, the sol-gel process is generally employed only for the purpose of producing precursors to crystalline powders upon heat-treatment. It is not suitable for producing amorphous products for optical purposes. More specifically, these defects are found to be more abundant in materials having at least two compositions except for silica (SiO.sub.2) so that it is practically impossible to fabricate a useful optical lens by the sol-gel process. Accordingly, desirable mechanical characteristics required for such an optical lens are often obtained from processing the polymers of organic compounds rather than by sol-gel processing the inorganic compounds.
In an exemplary organic polymerization method, a peroxide as an initiator is added to methyl methacrylate (MMA) as a monomer to facilitate polymerization between double bonds of the monomer, affording organic polymers of chain type, polymethylmethacrylate (PMMA). MMA is in liquid phase at ambient temperature under ambient pressure so that it acts both as a monomer as well as a solvent for polymerization. In addition, the organic polymer is usually a homopolymer consisting of only one material so long as the polymer is not synthesized by copolymerization. Thus, it is difficult to add a guest material to the organic polymer and to control the mechanical characteristics of the organic polymer. In particular, when ions having optical characteristics or macromolecules are added to the organic polymer, only a portion of the structure of the organic polymer adjacent to the guest material is restrictively modified while other surrounding portions of the polymer structure remain unchanged.
Such defects may be overcome by producing new materials with characteristics of both the parent inorganic and organic compounds and research on producing composites of this type has been actively pursued. The composites prepared from hybrid gels composed of organic and inorganic compounds including complex components of organic and inorganic polymers are referred to as an ORMOCER or a CFRAMER.
Generally, the composite is prepared by chemically reacting inorganic portions with the organic portions and reforming the inorganic components in situ. First, an alkoxide functional group of the metal alkoxide being a reactant in the inorganic polymerization step is partially substituted with organic monomers. Thereafter, the inorganic portions react with the organic portions via the substituted functional group.
Alternatively, research has been conducted to produce composites of organic and inorganic compounds without the aforementioned chemical treatment. An alcogel from the sol-gel process is first dried for a long time to obtain a dried gel and, then, an organic monomer is impregnated into the rigid metal oxide lattice of the dried gel to facilitate organic polymerization therein. In this process, the term alcogel refers to a gel that is impregnated with alcohol. However, the method using the alcogel has certain limitations in that the alcogel should be dried for several months so as to obtain a useful gel. Furthermore, the yield of the gel is extremely low.
In order to avoid such disadvantages while reducing the drying time, a crucial drying step is employed accompanying the sol-gel process to produce composites of organic and inorganic compounds in a relatively short time. However, it is known that the method cannot be applied to organic polymers such as PMMA which can not chemically bond to the inorganic oxide lattice.
Moreover, an investigation been made to combine an organic polymer with an inorganic oxide lattice structure. However, because the polymer is composed of long chains, it is not easy to produce the type of composites described herein.